High-reactance ballast transformer



Patented Aug. 17, 1954 ass-acts HIGH-REACTANCE BALLAST TRANSFORMERAndrew Schcvtchuk, Fort Wayne, Ind., assignor to General ElectricCompany, a corporation of New York Application November 26, 1952, SerialNo. 322,744

8 Claims. 1

This invention relates to stationary electrical induction apparatus andmore particularly to high reactance ballast transformers for use withfluorescent lamps.

Fluorescent lamps are are discharge devices and thus their internalresistance decreases as the current flow therethrough increases. It istherefore necessary in the installation of fluorescent lamps to providea current limiting device to limit the current flow through the lamp toa safe value. In addition, it may also be desirable to provide a highervoltage for initially starting or causing a discharge in the lamp thanis conveniently available. When fluorescent lamps are operated onalternating current, it is conventional practice to provide a singlehigh reactance ballast transformer to perform these two functions, i.e., current limiting and the provision of a high initial startingvoltage. Such a transformer provides a high open circuit voltage forstarting and also, by virtue of its high reactance, provides impedanceduring normal operation to limit the current flow.

A high reactance transformer is one in which the primary and secondarywindings are loosely coupled so that when the transformer is carryingload, there is a substantial amount of leakage flux and the voltagesinduced in the windings by this leakage flux cause the transformer tohave a relatively high elfective series reactance or What is commonlyknown as leakage reactance. The conventional way of constructing such atransformer is to arrange the primary and secondary windings ondifferent parts of the magnetic core and to provide a magnetic shuntbetween the windings thereby providing a path for the leakage fiux. Thisshunt is usually provided with an air gap so calibrated that the opencircuit secondary winding voltage of the transformer will be limited toa desired value. It is possible to operate two fluorescent lamps from asingle ballast by providing two secondary windings with a common primarywinding. In such installations, it is conventional practice to insert acapacitor in circuit with one of the lamps thereby causing that lamp todraw leading current whereas the other lamp will draw a lagging currentdue to a leakage reactance and inductive reactance of the secondarywinding to which it is connected. Operation of one lamp with leadingcurrent and the other lamp with lagging current improves the over-allpower factor of the ballast and further, when the two lamps arephysically located close together, the stroboscopic effect of the lightproduced by the lamps is materially decreased.

It is well known that lagging current flowing in a transformer secondarywinding will produce leakage flux which in effect bucks the fluxproduced by the primary winding so that the net output voltage of alagging current secondary under load is lower than the open circuitinduced voltage. In the case of a secondary winding having leadingcurrent flowing therein, however, the leakage flux produced thereby aidsthe primary flux thereby producing a higher output voltage under loadthan is produced under open circuit conditions. It is thereforenecessary to design the ballast to accommodate the increase in voltagein the leading secondary Winding with out saturating the core. This hasbeen accomplished in the past by placing a series air gap in the centerleg of the core in order to limit the amount of primary flux produced bythe primary Winding. Thus, if suflicient turns are wound on the core sothat the core does not saturate under load, the induced voltage in thesecondary windings under open circuit conditions will not be excessivesince the series gap will limit the primary flux. Another method ofaccommodating the high level flux in the leading secondary region of thecore is to operate the core under the pri mary coil and lead coil atsuch a low flux density under open circuit conditions that the inducedopen circuit secondary voltages are not excessive and the core hassufficient flux capacity in the region of the primary winding and leadsecondary winding for the flux produced under normal running conditions.Such an arrangement is shown in application Serial No. 155,226, filedApril 1'1, 1951, of Wesley W. Brooks and Andrew Schevtchuk. assigned tothe assignee of the present application, and now abandoned. Theobjection to the construction of this type is the fact that the core isnot symmetrical thereby making assembly more difficult. It is desirabletherefore to provide a symmetrical high reactance ballast transformerwhich will accommodate the flux in the region of the lead secondarywinding under operating conditions and which is easy to manufacture.

It is therefore an object of this invention to provide an improvedballast transformer incorporating the desirable features set forthabove.

Another object of this invention is to provide improved core for a highreactance ballast transformer designed to accommodate the flux in theregion of a leading current secondary winding.

A further object of this invention is to provide an improved fluorescentlamp circuit utilizing a ballast providing the desirable featuresenumerated above.

Further objects and advantages of this invention will become apparentand invention will be better understood by reference to the followingdescription and the accompanying drawing, and the features of noveltywhich characterize this invention will be pointed out with particularityin the claims annexed to and forming a part of this specification.

In accordance with this invention, a high reactance ballast transformeris provided for a two-lamp fluorescent circuit of the type having aleading current lamp and a lagging current lamp, in which the primaryflux produced by the primary winding is caused to follow parallel paths,one path linking with one secondary winding and the other path linkingwith another secondary winding. The core of the ballast is furtherarranged so that the leakage flux produced by each secondary winding issegregated from the two parallel paths of primary flux and from theleakage flux of the other winding. In this way, the primary and leakageflux in the region of the lead secondary winding is accommodated andeither secondary winding may be connected as a leading current winding.

In the drawing,'Fig. 1 is a side elevation of view, partly in section,illustrating the improved high reactance ballast transformer of thisinvention; and

Fig. 2 is a schematic illustration of a two-lamp ballast circuitutilizing the improved ballast transformer of this invention.

Referring now to the drawing, there is shown a magnetic core generallyidentified as I having a straight center winding leg 2. Center leg 2 islongitudinally split into two sections 3 and i defining a longitudinalair gap 5 therebetween. Two E-Shaped yoke members 6 and l are providedarranged respectively on either side of the center leg member 2, as willbe hereinafter more fully described. All of the members comprising thecore i are formed of a plurality of relatively thin laminations ofmagnetic material. The air gap 5 is maintained and the yoke members 6and l are held in assembled relation by suitable end clamps well knownin the art (not shown). While the center leg 2 of Fig. l is shown asincorporating longitudinal air gap 5, the center le member 2 may beformed as a single part thus omitting the longitudinal air gap 5.

The lower yoke 6 is provided with a first end leg 8 having a butt jointwith end S of center leg member segment 4 and another end leg iiidefining an air gap II with the other end l2 of center leg membersegment i. End legs 8 and it are joined by a bridging portion [3 and aleakage leg i4 is formed on bridging portion 83 forming a butt jointwith center leg segment i. Leakage leg I4 is displaced from thetransverse center line of the core thus defining a single coil receivingwindow [5 with end leg It and a double coil receiving window It with endleg t.

The upper yoke member i is arranged oppositely to the lower yoke B, i.e., with one end leg li forming a butt joint with end l8 of center legmember segment 3 and with its other end leg iii forming an air gap 26with end 2! of center leg member segment 3. End legs I1 and, [9 of upperyoke members i are joined by bridging portion 22 with leakage leg 23forming a butt joint with center leg member segment 3. Here again,leakage leg 23 is displaced from the transverse center 1ine of thedevice thereby forming a single coi1 receiving window 2,6 with end legis and a double coil receiving window 25 with end leg l1. It is thusclearly seen that end leg 8 of lower yoke member 6 abuts one end ofcenter leg member 2 while end leg ll of the upper yoke member 'i abutsthe other end of center leg member '2 and that end leg Hi of lower yokemember 6 forms an air gap with the end of center leg member 2 which isabutted by end leg ii and end leg it? forms an air gap with the end ofcenter leg member 2 abutted by end leg 8. It will also be seen that thedouble coil receiving window it or" lower yoke member 6 overlaps thedouble coil receiving window 25 of upper yoke member '5 while the singlecoil receiving windows 15 and 25 respectively overlap double coilreceiving windows 25 and I6.

Three electrical windings or coils are positioned on center leg 2including primary winding 26 and secondary windings 2? and 28, It willbe readily seen that primary winding 26 is positioned in both doublecoil receiving windows It and 25 while secondary winding 2? ispositioned in single coil receiving window it and double coil receivingwindow 25 and the other secondary winding 28 is positioned in singlecoil receiving window 24% and double coil receiving window It. Referringbriefly to Fig. 2, it will be seen that primary winding 28 is adapted tobe connected to an externa1 source of alternating current (not shown) bylines 29 and 3B. A first fluorescent lamp Si is connected across primarwindin 25 and secondary winding 28 by means of lines 32 and 33, thusmaking secondary winding 28 the lagging current secondary and otherfluorescent lamp 3a is connected across primary winding 26 and secondarywinding 21 by lines 32 and 35. A suitable capacitor 36 is arranged inseries with line 35 thus making winding 21 the leading current secondarywinding. As will be pointed out hereinafter, capacitor 3% could havebeen inserted in line 33 rather than line 35 since either secondarywinding 21 Or secondary winding 23 may serve as the leading currentsecondary winding.

Assuming now that primary winding 23 is connected to a source. ofalternating current across lines 29 and 30 and that lamps 3i and are notconnected to secondary windings 2i and i. e., the open circuitcondition, and referring again to Fig. 1, the primary flux dividesequally between two parallel magnetic paths, one being shown by arrows31 and the other by arrows 323. It will be seen that the primary flux inthe upper yoke section I proceeds down in leg ll into center leg section3, thence up into leakage leg 23 and then into bridging member 22 backinto end leg H. No appreciable amount of this primary flux will link theportion of secondary winding 28 occupying single coil window at byvirtue of the presence of air gap 2% between end leg i9 and single legmember segment It will be seen however that the primary flux 3'3 doelink that portion of secondary winding 2'! positioned in double coilwindow 25. The primary flux 38 enters end leg 8 of lower yoke member isfrom single leg member segment i passing next into bridging portion l3and thence through leakage leg I l back into center leg member segmentd. Here again, the primary flux 3? does not link the portion ofsecondary winding 27 positioned in single coil window [5 because of thepresence of air gap H between end leg It and single leg segment i.Primary flux 33, does however link the portion of secondary winding 28,positioned in double coil window It. It will now be seen that the twoparallel paths of primary flux respectively link each of the secondarywindings, primary flux 3'! linking the portion of secondary winding 21positioned in double coil window 25 and primary flux 38 linking theportion of secondary winding 28 positioned in double coil window IS.

The parallel primary fluxes 3i and 38 induce sufficient voltages insecondary coils 2'! and 28 to provide for starting the lamps 3i and 34.As oon as the lamps fire however, load current is drawn through thesecondary windings 21 and 28. In the leading lamp circuit, the voltageacross coil 21 increases as the lead lamp current flows through the coiland this current produces leakage flux. This flux takes two paths, thatshown by arrows 39 in upper yoke member 7 and center leg member segment3 and by arrows 4c in lower yoke member 6 and center leg member segment4. Here again it is seen that the leakage flux is split in two paths,the leakage flux 39 following the same path and the same direction asthe primary flux 3'! while the flux 40 follows a new path throughbridging portion I3 and leg ill across air gap II and through center legmember segment 4 back into leakage leg is and bridging portion I3. It isthus seen that only a portion of the leakage flux produced by leadsecondary winding 21 occupies the same portion of the core as theprimary flux 3'! thus permitting the use of less iron in the core thanwould be required to accommodate both lead leakage flux and primary fluxif both fluxes were required to traverse the same magnetic section.Furthermore, the provision of the air gap II in the magnetic path oflead leakage flux 40 causes the lead secondary winding 21 to act as areactor thus controlling the current in the circuit of lead lamp 3a.

In the circuit of lag lamp 3 I, the action is sim-- ilar although thevoltage across lag secondary coil 28 is not as high as the voltageacross lead secondary coil 21. Here, the leakage flux produced by lagsecondary winding 28 again follows two paths, one indicated by arrows 4iflowing in the same path as but bucking the primary flux 38 and theother shown by the arrows 42 flowing in a path previously unoccupied byflux, i. e., through center leg member segment 3, leakage leg 23,bridging portion 22, end leg l9 and finally air gap 20 back into centerleg segment Here again the provision of air gap 28 in the path ofleakage flux 42 serves to limit the current to the lag lamp 3|.

As pointed out above, the longitudinal air gap in the center leg 2 isnot necessary for satisfactory operation and thus a solid laminatedcen-- ter leg can be provided instead of the split center leg shown inFig. 1. It will also be apparent that this improved ballasttransformermay be used in series lead circuits as well as lead-lag circuits.

While I have shown and described a specific embodiment of thisinvention, further modifications and improvements will occur to thoseskilled in the art and I therefore intend in the appended claims tocover all modifications which do not depart from the spirit and scope ofthis invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A high reactance ballast transformer for two fluorescent lampscomprising a magnetic core having a straight center winding leg and twoE-shaped yoke members each having two end legs and a leakage legintermediate said end legs defining two coil windows, and a primary coiland two secondary coils on said center leg, said primary coil beingpositioned in a first coil window of each of said yoke members, one ofsaid secondary coils being positioned in the first coil window of one ofsaid yoke members and the other of said secondary coils being positionedin the first coil window of the other said yoke members whereby theprimary flux divides in parallel paths each linking one secondary coil,said one sec ondary coil being also positioned in the second coil windowof said other yoke member and said other secondary coil being alsopositioned in the second coil window of said one yoke member whereby theleakage flux of each of said secondary coils is isolated from saidprimary flux and from the leakage flux of the other of said secondarycoils.

2. A high reactance ballast transformer for two fluorescent lampscomprising a magnetic core having a straight center winding leg and twoE-shaped yoke members each having two end legs and a leakage legintermediate said end legs defining two coil windows, and a primary coiland two secondary coils on said center leg, said primary cell beingpositioned in a first coil window of each or" said yoke members, one ofsaid secondary coils being positioned in the first coil window of one ofsaid yoke members and the other of said secondary coils being positionedin the first coil window of the other said yoke members whereby theprimary flux divide in parallel paths each linking one secondary coil,said one secondary coil being also positioned in the second coil windowof said other yoke member and said other secondary coil being alsopositioned in the second coil window of said one yoke member whereby theleakage flux of each of said secondary coils is isolated from saidprimary flux and from the leakage flux of the other of said secondarycoils, one end leg of one of said yoke members defining an air gap withone end of said center leg and one end leg of the other of said yokemembers defining another air gap with the other end of said center legthereby causing said secondary coils to act as reactors.

3. In a high reactance ballast transformer for two fluorescent lamps, amagnetic core having a straight center winding leg and two iii-shapedyoke members, one of said yoke members having one or its end legsabutting one end of said center leg and the other of its end legsdefining an air gap with the other end of said center leg, the other ofsaid yoke members having one of its end legs abutting said other end ofsaid center leg and its other end leg defining another air gap with saidone end of said center leg, each of said yoke members having a leakagegap leg abutting said center leg and defining a single coil receivingwindow with said air gap defining end leg and a double coil receivingwindow with said abutting end leg.

4. In a high reactance ballast transformer for two fluorescent lamps, amagnetic core having a straight center winding leg and two E-shaped yokemembers, one of said yoke members having one of its end legs abuttingone end of said center leg and the other or its end legs defining an airgap with the other end of said center leg, the other of said yokemembers having one of its end legs abutting said other end of saidcenter leg and its other end leg defining another air gap with said oneend of said center leg, each of said yoke members having a leakage legabutting said center leg and defining a single coil receiving windowwith said air gap defining end leg and a double coil receiving windowwith said abutting end leg, each of said yoke members being disposedwith its single coil receiving window overlapping one end of a doublecoil receiving window of the other of said yoke members and with theother end of its double coil receiving windowoverlapping the other endof the double coil receiving window of the other of said yoke memberswhereby a primary coil may be arranged on said center eg and positionedin each of said double coil receiving windows and two secondary coilsmay be arranged on said center leg and respectively positioned in onesingle coil receiving window and one double coil receiving window.

5. In a high reactance ballast transformer for two fluorescent lamps, amagnetic core having a straight center winding leg and two E-shaped yokemembers, said center leg being longitudinally split into two spacedapart segments defining a longitudinal air gap therebetween, one of saidyoke members having one of its end legs abutting one of said center legsegments jacent one end or sa d center leg and the other or its endlegs, de hing an air gap with said one center leg segnent adjacent theother end of said center lei the other oi said yoke members having oneof its end legs abutting said other center leg segment adjacent saidother end of said center leg and its other end leg defining another hother center leg segment adjacent one end of said center leg, each oryoke members having a leakage respectively abutting said center legsegments and defining a s ngle coil receiving window with said air gapde; hing end leg and a double coil receiving window with said abuttingend leg.

6. A high reactance ballast transformer for two fluorescent comprising amagnetic core having a straight center winding leg and two E-shaped yokemembers, one of said yoke memberg having one of its end legs abuttingone end of said center leg and the other of its end legs defining an airgap with the other end of said center leg, the other of said yokemembers having one of its end legs abutting said other of said centerleg and its other end leg defining another air gap with said one end ofsaid center leg, each of said yoke members having a leakage leg abuttingsaid center leg and defining a single coil receiving window with saidair gap defining end leg and a double coil receiving window with saidabutting end leg, a primary coil on said center leg and adapted to beconnected to external source of alternating cur rent, said p 'inary coilbeing positioned in said double coil receiving window of both of saidyoke members, a first secondary coil on said center leg positioned insaid single coil receiving window of said one of said yoke members andin said double coil receiving window of said other yoke member, and asecond secondary coil on said leg positioned in said single coilreceiving window of said other yoke member and in said double coilwindow of said one yoke member.

'7. A high reactance ballast transformer for two fluorescent lampscomprising a magnetic core having a straight center winding leg and twoE-shaped yoke members, one of said yoke members having one of its endlegs abutting one end of said center leg and the other of its end legsdefining an air gap with the other end of said center leg, other of saidyoke members hav- 8: ing one of its end legs. abutting said other end ofsaid center leg and its other end leg defining another air gap with saidone end of said center leg, each of said, yoke members having a leakageleg abutting said center leg and defining a single coil receiving windowwith said air gap defining end leg and a double coil receiving windowwith said abutting end leg, each of said yoke membersv being disposedwith its single coil receivwindow overlapping one end of the double coilreceiving window of the other of said yoke members and with the otherend of its double coil receiving window overlapping the other end ofthe. double coil receiving window of the other of said yoke members, aprimary coil on said center leg and adapted to be connected to an,

external source of alternating current, said primary coil beingpositioned in said double coil receiving windows of both of said yokemembers, a first secondary coil on said center leg positioned in saidsingle coil receiving window of said one of said yoke. members and insaid double coil receiving window of said other yoke member, and asecond secondary coil on said center leg positioned in said single coilreceiving window or said other yoke member and in said double coilreceiving window of said one yoke member.

8. A high reactance ballast transformer for two fluorescent lampscomprising a magnetic core having a straight center winding leg and twoE-shaped yoke members, said center leg being longitudinally split intotwo spaced apart segments defining a longitudinal air gap therebetween,one of said yoke members having one of its end legsv abutting one ofsaid center leg segments adjacent one end of said center leg and theother of its end legs defining an air gap with said one center legsegment adjacent the other end of said center leg, the other of saidyoke members having one of its end legs abutting said other center legsegment adjacent said other end of said center leg and its other end legdefining another air gap with said other center leg segment adjacentsaid one end of said center leg, each of said yoke members having aleakage leg respectively abutting said center leg segments and defininga single coil receiving window with said air gap defining end leg and adouble coil receiving window with said abutting end leg, a primary coilon said center leg and adapted to be connected to an external source ofalternating current, saidprimary coil being positioned in said doublecoil receiving window of both of said yoke members, a first secondarycoil on sai center leg positioned in said single coil receiving windowof said one of said yoke members and in said double coil receivingwindow of said other yoke member, and a second secondary coil on saidcenter leg positioned in said single coil receiving window of said otheryoke member and in said double coil receiving window of said one yokemember.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,496,981 Boucher et al Feb. 7, 1950 2,578,395 Brooks Dec. 11,1951

